Many types of meters have been developed for measuring the flow of a medium within a pipe or other flow passage. One common type of flow meter uses a sensor external to the flow passage for responding to movement of a part within the flow passage to produce an electrical signal from which a representation of flow rate is developed.
U.S. Pat. No. 3,729,996 discloses a flow meter that includes a shaft which rotates within the flow passage at a rate proportional to the rate of fluid flow. A second shaft outside the flow passage and magnetically coupled to the first shaft rotates with the first shaft. An electrical circuit converts the rotation rate of the second shaft into a visual output.
In another type of flow meter, such as that disclosed in U.S. Pat. No. 4,507,976, Hall effect sensors respond directly to displacement of a magnet within the flow passage. The Hall effect sensor develops a signal based on the magnitude of the component of magnetic flux perpendicular to the face of the sensor, which varies with displacement of the magnet. As with the prior art discussed above, the design of this flow meter includes electrical circuitry which converts the output of the sensor into a visual representation of flow rate.